Vibration analysis has been used for years to provide a determination of the proper functioning of different types of machinery, including rotating machinery and rocket engines. A determination of a malfunction, if detected at a relatively early stage in its development, will allow changes in operating mode or a sequenced of the machinery prior to a total failure. Such preventative measures result in less extensive and/or less expensive repairs, and can also prevent a sometimes catastrophic failure of equipment. In a rocket thruster, for instance, certain frequencies can be calculated as a function of the geometry of the thruster dimensions and, if detected, indicate thruster instability. If thruster vibration is monitored with respect to these critical frequencies or is detected in bands of frequencies around the critical frequencies, then it is possible to either change operating modes or provide an orderly shutdown of the thruster prior to failure. A change of operating mode for a thruster may include changing control signals to the thruster from a pulsed mode to a continuous mode signal. If such a change in operating mode is not successful to eliminate vibrations at a critical frequency, then it becomes necessary to shut down the thruster.
Accelerometers which are commercially available are contained within a package that houses only the transducer itself and may include wiring to a connector. In application, this packaging presents a number of problems. The signal wire from the accelerometer will typically go through a connector and then make its way through an electrically noisy environment before reaching a second connector, which will take the signal to a detector circuit. Noise may be picked up on this signal wire and cause false machine instability signals. Also, because the detector circuitry and accelerometer transducer may be located some distance apart and may be powered by different power supplies, ground loop errors may occur. Ground loops or ground shifts are differences between circuit grounds which may occur when circuits are powered by different power supplies, or are powered by the same power supply when the circuits are located a considerable distance apart. Relatively higher currents carried by one part of a grounding system can also result in voltage differences between other parts of a grounding system. These voltage differences between grounds, which are often called ground shifts or ground loops, are typically variable, unknown, and insidiously cause false signals. When adding vibration monitoring and detecting circuitry it may be difficult or impossible to determine whether ground loops exist in other circuitry which may cause errors or false signals in the vibration monitoring apparatus. Similarly, noise and ground loop problems may occur when connecting an instability or fault signal wire from the detection circuitry of a vibration monitoring apparatus to a distant control circuit or computer which utilizes the fault signal to supervise a machine shut down. Typically, vibrating machinery will produce a noisy electrical environment. As well, the vibration may produce occasional changes in electrical resistance in contacts, connectors, etc. which may produce spurious machinery fault signals.
R. J. Bozeman U.S. Pat. No. 4,977,395 discloses an adjustable delay circuit which can be used to delay the output of an instability signal until the particular type of vibration signal which denotes instability has occurred consistently enough over a period of time (e.g. 40 msec for a rocket thruster) to indicate an instability signal which is more likely to be valid. However, adding this delay circuit increases overall circuit complexity which increases the possibility of a circuit failure, and requires additional room. Also, the output of the delay circuit is itself subject to the noise and ground loop problems so that its purpose may still be defeated even if it is effective to distinguish spurious signals up to the input of the delay circuit.
Consequently, a need exists for improvements in accelerometer circuitry to provide a highly reliable means for preventing false signals from the accelerometer transducers and related circuitry operating in a vibrating, electrically harsh environment. Those skilled in the art have long sought and will appreciate the novel features of the present invention which solves these problems.